1. Field of the Invention
This invention relates to a liquid crystal display panel, and more particularly to structures within a liquid crystal display panel with a touch panel to minimize light deterioration and image distortion.
2. Description of the Related Art
A conventional liquid crystal display panel controls a light transmissivity of liquid crystal cells to display a picture. Liquid crystal cells are formed between two glass substrates. Each liquid crystal cell responds to a video signal, i.e. a pixel signal, to control the transmitted light quantity.
Such a liquid crystal display panel maybe mounted with a touch panel to be used as an input device. A user touches the touch panel through a stylus or a finger to input instructions or information. The touch panel generates a voltage or current signal corresponding to a position where the touch panel is touched.
The touch panel can be either a capacitive or a resistive type. FIG. 1 shows a liquid crystal display panel 10 mounted with a capacitive touch panel 12 and FIG. 2 shows the same liquid crystal display panel 10 mounted with a resistive touch panel 16.
Referring to FIG. 1, the liquid crystal display panel 10 includes upper and lower polarizing sheets 14A and 14B. Upper and lower glass substrates 20B and 20A are positioned below and above the upper and lower polarizing sheets 14A and 14B, respectively. Gate lines 23, insulating film 24, pixel electrodes 25A, and a first orientation film 26A are sequentially provided above the lower glass substrate 20A. Below the upper glass substrate 20B, a black matrix 27, color filter 28, a common electrode 25B, and a second orientation film 26B are sequentially provided.
Finally, ball spacers 22 and a liquid crystal material 21 are disposed between the first and second orientation films 26A and 26B. The purpose of the ball spacers 22 is to keep the distance between the upper and lower glass substrates 20B and 20A as uniform as possible. This in turn keeps the thickness of the liquid crystal material 21 uniform as well.
Also referring to FIG. 1, the capacitive touch panel 12 includes a glass sheet 30, an electrode layer 31, and an insulating layer 32 sequentially disposed above the upper polarizing sheet 14A. The electrode layer 31 serves as a dielectric layer, and the insulating layer 32 prevents an electrical short from occurring when the touch panel 12 is pressurized by an input device such as a stylus or a finger (not shown).
When the touch panel 12 is pressurized, a capacitance value is changed at the pressurized point. The electrode layer 31 detects the capacitance value and generates either a current or a voltage signal corresponding to the capacitance value.
Referring to FIG. 2, the liquid crystal display panel 10 is the same as the display panel described in FIG. 1. Thus, detailed description of the display panel 10 is omitted. The touch panel 16 of FIG. 2 is resistive. The resistive touch panel 16 includes a glass sheet 33, first and second electrode layers 36A and 38B above the glass sheet 33, touch panel spacers 35 disposed in between the first and second electrode layers 36A and 36B, and an insulating sheet 34 above the second electrode layer 36B.
When the touch panel 16 is pressurized, a short is created between the first and second electrode layers 36A and 36B at the pressurized point, which generates differing current or voltage corresponding to the pressurized point.
As mentioned above, ball spacers 22 are used to maintain a uniform distance between the upper and lower glass substrates 20B and 20A in the liquid crystal display panel 10 of FIGS. 1 and 2. However, the display image becomes distorted at the points where the touch panel 12 or 16 is pressurized, for example by a stylus or a finger.
More specifically, when the stylus is used on the touch panel 12 or 16, the upper glass substrate 20B is also pushed corresponding to the position of the stylus on the touch panel. Then the distance between the upper and lower glass substrates 20B and 20A is locally narrowed, and thus the electric field intensity applied to the liquid crystal material 21 is locally changed as well. This changes the amount of light transmitted around the pressurized area.
Another contributing factor to the distortion is that the pressure created by the stylus or the finger cannot sufficiently be absorbed due to the ball spacers 22 making point contacts with the upper and lower glass substrates 20B and 20A.
Further, the amount of distortion varies depending on the position of the pressurized point. This is because ball spacers 22 are not uniformly distributed. As shown in FIG. 3, the density of ball spacers 22 are not uniformly among the different pixel electrodes 25A on the lower glass substrate 20A. As a result, amounts of light deterioration and image distortion are different according to the position of the pressurized point.
To prevent image distortion as described above, it has been suggested to densely spread the ball spacers 22. However, distortion still occurs even if the balls spacers are more densely packed. For example, FIGS. 4A and 4B show a display panel where the ball spacer density is increased by three times as described in the above conventional art. It is shown that light deterioration is still generated on the touch panel 12 or 16 at the position pushed by the stylus. Due to the light deterioration, a distorted image with a wave shape is observed around the pressurized position.
FIG. 5 shows a liquid crystal display panel according to a Japanese Laid-Open Patent Gazette No. 1985-182414, which has spacers patterned differently from those shown in FIGS. 1 and 2. FIG. 5 is a top view of a lower glass substrate 40 that include gate lines 41 and data lines 42 arranged to cross each other. Pixel electrodes 43 are positioned at cell areas defined by the gate and data lines 41 and 42.
Thin film transistors (TFTs) 44 are provided at each intersection between the gate and data lines 41 and 42 and serve as switches. A column of TFTs 44 respond to a signal from the gate line 41 to selectively connect a column of pixel electrodes 43 to the data lines 42, which are then used to input display information to each selected pixel electrode.
Patterned spacers 45 are formed on TFTs 44. Like the ball spacers 22 of FIGS. 1 and 2, the patterned spacers are formed to keep a uniform distance between the upper glass substrate (not shown in FIG. 5) and lower glass substrate 40. However, unlike the ball spacers of the previous figures, the patterned spacers 45 are overlapped with each pixel electrode 43. This allows the liquid crystal display panel to display a high quality picture.
However, even the liquid crystal display panel as shown in FIG. 5 has difficulty in keeping a uniform distance between the upper and lower glass substrates. This is because, like the ball spacers 22, the contact area of the spacers contacting the upper glass substrate is small. Because of the small contact area, the display panel cannot sufficiently absorb the pressure caused by a stylus or a finger. Thus like the display panels of FIGS. 1 and 2, local image distortion occurs, even for the display panel shown in FIG. 5.